In recent years "modular" thermal insulation devices have come into widespread use. These are blocks of thermal insulation fitted with means to attach them to the walls of furnaces and similar high temperature units. The modules or blocks usually have about 1 ft.sup.2 (930 cm.sup.2) faces and have an insulation material depth of from 4 to 12 inches (10 to 30 cm). A typical module or block is shown in U.S. Pat. No. 4,001,996 to C.O. Byrd, Jr.; modules of this type are commercially available under the trademark "Z-BLOK" from the Johns-Manville Corporation and its licensees.
All of the various prior art devices of this modular type have consisted of single layers of insulating fiber, and the fiber depth is obtained by folding the fiber as shown in the aforementioned Byrd patent or by having straight fibers of predetermined lengths, such as shown in U.S. Pat. No. 3,832,815. Varying the depth of the single layer of fiber suffices for many different types of insulation requirements, so that the desired temperature drop from the hot face of the blanket to the cold face is obtained. Because there is only a single fiber layer, however, the module must be constructed throughout with fiber which can withstand the hot face temperature. For lower temperature service where relatively inexpensive fibrous materials provide adequate insulation, this is not a particularly serious detriment. Where the hot face temperature is above about 1200.degree. F. (650.degree. C.) and particularly where it is above about 1800.degree. F. (980.degree. C.), the limitations of single layer construction become much more evident. Fibrous materials designed to withstand these high hot face temperatures must be formed from quite pure raw materials and under rather demanding formation conditions, and consequently are quite high in cost. Because there is normally a substantial temperature drop across the depth of a fiber insulating module (which temperature drop is greater the greater the depth of the module), the cold face side of the module normally does not require such high temperature service properties in the fiber. However, since the block is made of only a single type of fiber, the expensive high temperature resistant fiber must be used for the entire block. This effectively wastes costly fiber at the back of the module where its properties are not needed and significantly adds to the cost of the finished module.
Attempts have been made to overcome this problem by attaching high temperature fiber layers to the hot face of the blocks by various complex mechanical means; see, e.g., U.S. Pat. Nos. 4,055,926; 4,086,737; 4,103,469 and 4,123,886, all to the aforementioned C.O. Byrd, Jr.
It would therefore be of considerable interest to have available a modular or block thermal insulating device which would permit one to utilize high temperature resistant fiber at the hot face thereof and fiber of lesser temperature resistance at the cold face thereof, while providing for a simple means of securing the layers of fiber together.